1. Field of the Invention
The present invention relates to digital image processing and, more particularly, to processing color continuous-tone images into halftone images for ink-based printing.
2. Description of the Related Art
A color inkjet printer has the capability to render an image with color dots that are either bi-level or multi-level in intensity. Color dots of various intensities can be generated by using multiple chambers in print heads, each loading color inks with a different dye load. One example of such a printing system employs regular dye-load cyan, magenta, yellow, and black inks, and reduced dye-load cyan and magenta inks.
A printer can also achieve a higher scan axis printing resolution by increasing the firing frequency relative to the cartridge speed. Because the printer dot size remains the same, this is often referred to as the printer addressability. For example, a 300 dpi (dot per inch) printer can have the addressability of 600.times.300 dpi.
Image Resolution Enhancement Technology (IRET) includes two components: a multi-level halftoning algorithm, and basic pattern design. The multi-leveling halftoning algorithm is a general halftoning method, while the basic pattern design is more device specific. IRET has been successfully implemented in the Hewlett-Packard Color LaserJet laser printer. See, Q. Lin, Methods to Print N-Tone Images with Multi-leveling Techniques, U.S. patent application Ser. No. 08/205,672, filed Mar. 2, 1994, HP Docket 1093765-1; Q. Lin, Printing N-Tone Images with Imperceptible Dots, U.S. Pat. No. 5,615,021, issued Mar. 21, 1997, HP Docket 1094347-1; Q. Lin, B. Hoffmann, and J. Trask, Resolution Enhancement Procedure and Apparatus for a Color Laser Printer, U.S. patent application Ser. No. 08/788,767, filed Jan. 24, 1997, HP Docket 10960458-1.
It is desirable to develop a halftoning method for rendering a color image that achieves near-photographic image quality on a color inkjet printer having multiple dye loads and increased scan axis addressability. However, as will be discussed below in greater detail, one cannot readily extend the IRET to a dual-dye load inkjet printer. A standard application of IRET to a dual-dye load inkjet printer will yield images that have undesirable artifacts such as banding, texturing and "worms". Furthermore, a large amount of processing time will be required to render an image and an inordinate amount of ink will be used. Besides increasing the cost of printing, the use of excessive ink can cause additional undesirable effects such as an increased dry time for each print, curling of the paper, and smudging of the image.
Thus, it can be seen that physical properties of ink-based printing impose cost, output speed, and image fidelity limits upon ink-based color halftone image output devices, and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for a technique that can improve color halftone imaging for ink-based printing by reducing the amount of time necessary to perform the halftoning process, by reducing the amount of ink consumed and by increasing the quality of the images produced.